Re: [PATCH v2 2/3] virtiofs: export filesystem tags through sysfs

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Feb 09, 2024 at 10:36:47AM +0000, Greg KH wrote:
> On Thu, Feb 08, 2024 at 02:32:10PM -0500, Stefan Hajnoczi wrote:
> > The virtiofs filesystem is mounted using a "tag" which is exported by
> > the virtiofs device:
> > 
> >   # mount -t virtiofs <tag> /mnt
> > 
> > The virtiofs driver knows about all the available tags but these are
> > currently not exported to user space.
> > 
> > People have asked for these tags to be exported to user space. Most
> > recently Lennart Poettering has asked for it as he wants to scan the
> > tags and mount virtiofs automatically in certain cases.
> > 
> > https://gitlab.com/virtio-fs/virtiofsd/-/issues/128
> > 
> > This patch exports tags at /sys/fs/virtiofs/<N>/tag where N is the id of
> > the virtiofs device. The filesystem tag can be obtained by reading this
> > "tag" file.
> > 
> > There is also a symlink at /sys/fs/virtiofs/<N>/device that points to
> > the virtiofs device that exports this tag.
> > 
> > This patch converts the existing struct virtio_fs into a full kobject.
> > It already had a refcount so it's an easy change. The virtio_fs objects
> > can then be exposed in a kset at /sys/fs/virtiofs/. Note that virtio_fs
> > objects may live slightly longer than we wish for them to be exposed to
> > userspace, so kobject_del() is called explicitly when the underlying
> > virtio_device is removed. The virtio_fs object is freed when all
> > references are dropped (e.g. active mounts) but disappears as soon as
> > the virtiofs device is gone.
> > 
> > Originally-by: Vivek Goyal <vgoyal@redhat.com>
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >  fs/fuse/virtio_fs.c                         | 113 ++++++++++++++++----
> >  Documentation/ABI/testing/sysfs-fs-virtiofs |  11 ++
> >  2 files changed, 103 insertions(+), 21 deletions(-)
> >  create mode 100644 Documentation/ABI/testing/sysfs-fs-virtiofs
> > 
> > diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> > index de9a38efdf1e..28e96b7cde00 100644
> > --- a/fs/fuse/virtio_fs.c
> > +++ b/fs/fuse/virtio_fs.c
> > @@ -31,6 +31,9 @@
> >  static DEFINE_MUTEX(virtio_fs_mutex);
> >  static LIST_HEAD(virtio_fs_instances);
> >  
> > +/* The /sys/fs/virtio_fs/ kset */
> > +static struct kset *virtio_fs_kset;
> > +
> >  enum {
> >  	VQ_HIPRIO,
> >  	VQ_REQUEST
> > @@ -55,7 +58,7 @@ struct virtio_fs_vq {
> >  
> >  /* A virtio-fs device instance */
> >  struct virtio_fs {
> > -	struct kref refcount;
> > +	struct kobject kobj;
> >  	struct list_head list;    /* on virtio_fs_instances */
> >  	char *tag;
> >  	struct virtio_fs_vq *vqs;
> > @@ -161,18 +164,43 @@ static inline void dec_in_flight_req(struct virtio_fs_vq *fsvq)
> >  		complete(&fsvq->in_flight_zero);
> >  }
> >  
> > -static void release_virtio_fs_obj(struct kref *ref)
> > +static ssize_t virtio_fs_tag_attr_show(struct kobject *kobj,
> > +		struct kobj_attribute *attr, char *buf)
> >  {
> > -	struct virtio_fs *vfs = container_of(ref, struct virtio_fs, refcount);
> > +	struct virtio_fs *fs = container_of(kobj, struct virtio_fs, kobj);
> > +
> > +	return sysfs_emit(buf, fs->tag);
> > +}
> > +
> > +static struct kobj_attribute virtio_fs_tag_attr = {
> > +	.attr = { .name = "tag", .mode= 0644 },
> > +	.show = virtio_fs_tag_attr_show,
> > +};
> 
> __ATTR_RO()?
> That way we all know you got the mode setting correct :)
> 
> Other than that minor thing, looks good, nice job!

Will fix, thanks!

Stefan

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